Formation of vessels such as drawn polyester bottles is very popular at the present because they provide substantial weight advantage over glass bottles especially in shipping weight. These polyester vessels have excellent transparency and somewhat acceptable but not optimum gas-barrier properties, and they have been broadly used as vessels for liquid detergents, shampoos, cosmetics, and also for carbonated drinks such as beer, cola and soda pop and refreshing drinks such as fruit juices and mineral water.
The drawn polyester bottles exhibit permeability to oxygen, carbon dioxide gas and the like though the permeability is small, while the gas permeability of completely sealed vessels such as glass bottles and metal cans is substantially zero. Accordingly, drawn polyester bottles are inferior to cans and glass bottles especially in the case of carbonated drinks, where loss of carbon dioxide gas occurs and there is a definite limit to the storage period.
Polyethylene terephthalate and its copolyesters (hereinafter referred to collectively as “PET”) is the polyester of choice and is widely used to make containers for carbonated soft drinks, juice, water, and the like due to their excellent combination of clarity, mechanical, and somewhat acceptable gas barrier properties. In spite of these desirable characteristics, insufficient gas barrier of PET to oxygen and carbon dioxide limits the application of PET for smaller sized packages, as well as for packaging oxygen sensitive products, such as beer, juice, and tea products. A widely long felt need exists in the packaging industry to further improve the gas barrier properties of PET.
The relatively high permeability of PET to carbon dioxide limits the use of smaller PET containers for packaging carbonated soft drinks. The permeation rate of carbon dioxide through PET containers is in the range of 3 to 14 cc's per day or 1.5 to 2 percent per week loss rate at room temperature depending on the size of the container. A smaller container has a larger surface area to volume ratio resulting in a higher relative loss rate. For this reason, PET containers are currently used only as larger containers for packaging carbonated soft drinks, while metal cans and glass containers are the choice for smaller carbonated soft drink containers.
The amount of carbon dioxide that remains in a packaged carbonated soft drink determines its shelf life. Normally, carbonated soft drink containers are filled with approximately four volumes of carbon dioxide per volume of water. It is generally accepted in the industry that a packaged carbonated soft drink reaches the end of its shelf life when 17.5 percent of the carbon dioxide in the container is lost due to permeation of the carbon dioxide through the container side wall and closure. The permeability of PET to carbon dioxide therefore determines the shelf life of the packaged carbonated beverage and thus, the suitability of PET as a packaging material.
A wide variety of technologies have been developed or are being developed to enhance the barrier properties of PET to small gas molecules. For example, external or internal coatings for enhancing the gas barrier of PET containers have been developed. The coating layer is normally a very high barrier layer, either inorganic or organic, and slows down the diffusion of gases. Implementation of this technology, however, requires coating equipment not normally utilized in the manufacture of packaged beverages and therefore requires substantial capital investment, increased energy usage, and increased floor space. In many beverage packaging plants that are already crowded, the additional space is not an option.
Multi-layered containers have also been developed with a high barrier layer sandwiched between two or more PET layers. Implementation of this technology also requires substantial capital investment and delamination of the container layers impacts appearance, barrier, and mechanical performance of the containers.
A barrier additive for the PET or a polymer with inherent good barrier properties would be good solutions that are welcomed by the industry. Neither such solution requires additional capital investment, and therefore, does not have the limitations inherent with other technologies. A barrier additive can also be added during the injection molding process which gives more flexibility for downstream operations.
PET has been modified or blended with other components to enhance the gas barrier of the PET. Examples include polyethylene naphthalate (PEN)/PET copolymers or blends, isophthalate (IPA) modified PET, PET blended with polyethylene isophthalate (PEI) or a polyamide, such as nylon, and PET modified with resorcinol based diols. For a PET copolymer to achieve moderate barrier enhancement of 2× or higher, the modification is normally more than 10 to 20 weight or mole percent of the total co-monomers. When PET is modified to such a high level, the stretching characteristics of the PET are changed dramatically such that the normal PET container preform design could not be used in the manufacture of containers. Using these PET copolymers to mold conventional PET container preforms results in preforms that can not be fully stretched and the ultimate containers are very difficult, if not impossible, to make. Even if such a container can be made, it does not show improved barrier performance and shows deteriorated physical performance such that it can not be used to package carbonated soft drinks.
Furthermore, PET blends with polyamide such as nylon developed yellowness and haze and are not clear like conventional PET.
Accordingly, there is a long felt need in the art to enhance the barrier performance of PET for use in applications that will require enhanced barrier properties, such as in the packaging of carbonated beverages and oxygen sensitive beverages and foods, in a manner that does not cause substantial degradation of the PET, does not substantially impact the stretch ratio of the PET, and does not negatively impact the clarity of the PET.
Additionally, numerous examples of diesters of aromatic dicarboxylic acids have been disclosed in the prior art. For example, bis(hydroxyalkyl) esters of terephthalic and isophthalic acid are precursors for the preparation of poly(alkylene)arylates such as polyethylene terephthalate, polybutylene terphthalate, and polypropylene terephthalate. Simple diesters of phthalic acid or phthalic anhydride such as bis(2-ethylhexyl) phthalate are widely known as being effective plasticizers for a variety of plastics. Diesters of aromatic dicarboxylic acids such as dimethylterephthalate, diethylterephthalate and diphenylterephthalate have been demonstrated to be useful as barrier enhancing additives in aromatic polyesters such as polyethylene terephtahlate as disclosed in US patent application US 2006/0225568. Also, monoesters of hydroxybenzoic acid have also been disclosed that function as plasticizers in aromatic polyesters WO/01/12521.